This invention relates to simplified automatic means for feathering the propeller of a turboprop aircraft when there is a powerplant failure. The system was designed for feathering the pitch of a variable pitch propeller of the type using Beta control. Beta control simply implies a mode of operation wherein the pilot has direct control of the blade angle of the propeller while the fuel governor maintains the desired engine r.p.m. My invention was implemented on a quick-acting propeller having Beta pitch control through the center of the propeller drive shaft.
Other type of negative torque sensors have been built for use with aircraft gas turbine engines. U.S. Pat. No. 2,801,701 by Coar describes an automatic feathering control system whose applicability is limited to free turbine systems. Further, Coar requires an oil system independent of the engine oil system which is all that is needed with my invention. Thus, Coar needs special pumps, reservoirs and plumbing not required by my system. In U.S. Pat. No. 2,955,658 by Lombard, the negative torque sensing linkage is interconnected with the constant speed control unit. As a result, the control system is arranged so the reverse torque sensor cannot feather the propeller when the throttle lever is in a low power or idle setting.
The U.S. Pat. No. 2,959,228 by Larkin contemplates a much more complex control system than is desired for use on a general aviation type aircraft piloted by a single individual. In Larkin, automatic feathering of the propeller is intended for use on take-off only. In the cruise flight condition, arming and throttle control disable the automatic feathering feature. The torque sensing and control system of Larkin is dependent on an electrical source and will automatically feather a propeller only when there is an engine failure during the full power-take-off portion of the flight regime. Even then the circuit must have been pre-armed by the pilot. My invention, in contrast, requires no electrical power source since it functions hydraulically. Additionally, my system is always functional and automatically recycles to a neutral state after feathering the propeller.